As a cathode material, Li1.2Ni0.2Mn0.6O2 delivers high discharge capacity at low C-rate but low discharge capacity at high C-rate. Furthermore, a large potential hysteresis occurs during charge and discharge, and the charge-discharge curves change shape during the charge-discharge cycling. Increasing nickel/manganese (Ni/ Mn) ratio in Li1.2Ni0.2Mn0.6O2 improves discharge capacity at high C-rate, but it decreases discharge capacity at low C-rate. To accomplish high discharge capacity at both high and low C-rates, lithium/transition-metal (Li/ TM) ratio in high-nickel-content material, namely, Li1.2Ni0.2Mn0.6O2, was adjusted. Cathode materials with varied lithium/transition-metal ratio (x), namely,Li1.2Ni0.2Mn0.6O2 + (0.35/0.8)xMn(0.45) + (0.45/0.8)x02 = 0, 0.02, 0.04, 0.06, or 0.08), were prepared, and their electrochemical performances were evaluated. It was found that as x was decreased, discharge capacity first increased then decreased, and Li1.2Ni0.2Mn0.6O2+ (0.35/0.8)xMn(0.45) + (0.45/0.8).x02 (x = 0.04), which can be described as Li1.2Ni0.2Mn0.6O2, was found to exhibit the highest discharge capacity. Accordingly, the electrochemical properties of Li1.2Ni0.2Mn0.6O2 were compared with those of a conventional lithium-rich layer-structured cathode material, namely, 1.11.2Nlio.2Mno.602. The rate performance of Li1.2Ni0.2Mn0.6O2 was higher than that of Li1.2Ni0.2Mn0.6O2. Furthermore, potential hysteresis and shape change of the charge-discharge curves for Li1.2Ni0.2Mn0.6O2 were smaller than those for Li1.2Ni0.2Mn0.6O2. It is thus concluded that changing the composition of the cathode material from Li1.2Ni0.2Mn0.6O2 to Li1.2Ni0.2Mn0.6O2 alleviates the drawbacks of lithium-rich layer-structured cathode material, namely, low rate performance, potential hysteresis, and shape change of charge-discharge curves.